Accueil > Équipes > Magnétisme Solaire et Stellaire > donati > MAPP : main scientific goals

 

MAPP : main scientific goals

Par Jean-Francois Donati - 18/04/2008

 

MAPP:
main scientific goals


© C Liljegren & D Dravins, Lund

Whereas the understanding of most phases of stellar evolution made considerable progress throughout the whole of the twentieth century, stellar formation remained rather enigmatic and poorly constrained by observations until about three decades ago. One of the major discoveries obtained at this time is that protostellar accretion discs are often associated with extremely powerful and highly collimated jets escaping the disc along its rotation axis (eg Burrows et al, 2006, ApJ 473, 437). This finding has revolutionized the field of stellar formation; in particular, it very strongly suggested that magnetic fields are playing a major role throughout stellar formation (Pudritz & Norman, 1983, ApJ 274, 677).

MAPP aims at studying in detail the role of magnetic fields in stellar formation. Youth is indeed the period in the life of stars where magnetic fields are expected to have the strongest impact, eg through the magnetically-controlled accretion/ejection processes involved in the collapse of the protostellar cloud from which stars and planets form. By collecting new observational data and running dedicated multi-D MHD numerical simulations, MAPP is expected to unveil how magnetic fields modify the formation of stars and their protoplanetary systems.

MAPP focusses especially on the core regions of protostellar accretion discs - including the newly born star and closely orbiting planets - and concentrates on the following 4 main topics:

By carrying out Doppler tomography of protostellar accretion discs from time-resolved spectropolarimetric data sets, MAPP will produce charts of the density and magnetic field in the core regions of the accretion disc (eg Donati et al 2005, Nature 438, 466). By comparing with results from MHD simulations of molecular cloud collapse (eg Banerjee & Pudritz 2006, ApJ 641, 949; Hennebelle & Fromang 2007, A&A, in press), MAPP should reveal the origin of the magnetic field in the disc and indicate whether it is a fossil remnant or a dynamo output. It will also check whether the magnetic field is indeed capable of enhancing accretion through instabilities (eg Balbus & Hawley 2003, LNP 614, 329) and of inhibiting the formation of protostellar/protoplanetary clumps (eg Fromang 2005, A&A 441, 1; Hennebelle & Teyssier 2007, A&A, in press).

By using Zeeman-Doppler imaging on sets of spectropolarimetric observations of classical T Tauri stars (cTTs), MAPP will provide detailed views of magnetic structures at the surface of newly born stars (eg Donati et al 2007, MNRAS 380, 1297). These maps will tell us how the magnetic field connects the star to the accretion disc and how disc material is transferred to the star through magnetospheric accretion processes (eg Jardine et al 2006, MNRAS 367, 917; Gregory et al MNRAS 371, 999). MAPP will then explore how accretion processes and magnetic field impact the thermal structure of the forming star (eg Chabrier et al 2007, Protostars & Planets V; Chabrier et al 2007, A&A 472, L17) and how they affect the angular momentum history (eg Long et al 2006, ApJ 634, 1214).

From results on the star and disc magnetic structures, MAPP will test and update existing theoretical models of collimated jet and wind formation from young stars and/or their accretion discs (eg Ferreira et al 2006, A&A 453, 785). In particular, it will explore the role of magnetic fields in controlling the disc ability to fire jets and winds (eg Ménard & Duchène 2004, A&A 425, 973) and will quantify how much mass and angular momentum is extracted from the disc and/or the star through such processes.

Finally, MAPP will investigate the role of magnetic fields in the formation close-in giant planets through orbital migration, and will look for density gaps in the core regions of protostellar accretion discs and their relation with respect to the disc magnetic field. By doing so, MAPP should be able to conclude whether migrating giant planets are stopped by azimuthal magnetic fields in the disc (eg Terquem 2003, MNRAS 341, 1157) or by entring the central magnetospheric hole that the baby star has dug (eg Romanova & Lovelace 2006, ApJ 645, L73).

Image credits: Banerjee & Pudritz (middle) and Donati & Jardine (bottom)

 

 

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